1. Field of the Invention
This invention is generally related to bow compensation in raster output scanners. In particular, this invention is directed to a system for minimizing differential bow distortion of the beams in multi-beam raster output scanners used for single and multi-station xerography based electronic printers and copiers. More specifically, this invention ensures that both the shape and the positioning of any remaining bow in the multiple beams are identical between the beams.
2. Related Art
The basic functions of rotating polygon-based optical systems in general are set forth below, and are described only for easier understanding of the technical discussion set forth below and in the detailed description of the preferred embodiments. Prior art FIG. 5 shows a known rotating polygon multi-beam ROS scanner. It should be appreciated that the functions described below equally apply to most polygon-based systems, independently of number of light sources used.
FIG. 5 shows a pair of sagittally offset laser diodes 31 and 32. The beams 43 and 42 emitted by laser diodes 31 and 32 are collimated by the collimator 33 (lens L1). A sagittal aperture 34 is placed in the position where the beams 41 and 42 cross the optical axis, to control the F/#, which in turn controls the spot size. The input cylinder optical element 35 (lens L2) focuses the beams 41 and 42 on the surface of the current polygon facet 36 of the rotating polygon. After reflecting from the current facet 36, the beams 41 and 42 pass through the F.theta. lens 37 (lens L3). The F.theta. lens 37, in general, has relatively low power in the tangential meridian. The main function of the F.theta. lens 37 is to provide focusing in the tangential meridian and control the scan linearity, in terms of uniform spot displacement per unit angle of polygon rotation.
The function of the motion compensating optical element (MCO) 39 is to reimage the focused beams 41 and 42 from the current polygon facet 36 onto the Photo Receptor (PR) plane 40 at a predetermined position, independently of the polygon angle error or tilt of the current facet 36. Such compensation is possible because the focused beams are stationary "objects" for the F.theta. lens 37 and the MCO 39. Although, due to polygon tilt, or wobble, the beams 41 and 42 are reflected to different positions of the post-polygon optics aperture for each different facet of the rotating polygon, the beams 41 and 42 are imaged to the same position on the PR plane 40.
In rotating polygon, single spot ROS-based xerographic copiers and printers, bow distortions occur from the accumulation of optics tolerances. Bow itself is the curved line described by the scanned laser beam of the ROS as it moves in the fast scan direction. Thus, the bow appears as a displacement of a scan line in the process direction as the line extends in the fast scan direction.
Although multi-beam, laser diode based ROS is viewed as the most powerful technology for high quality, high throughput xerographic printing, the phenomenon known as differential scan line bow remains as an undesirable side affect. Differential scan line bow arises from the very nature of multi-beam optical systems, where the beams are offset sagittally (in the cross-scan direction) so that half of the beams lie above and half of the beams lie below, or all of the beams lie above or below, the optical axis.
Depending on the design of the system, the differential scan line bow can cause the scan lines to move toward each other (barrel distortion), or away from each other (pin cushion distortion). In both of these cases, the light sources (lasers) are placed on opposite sides of the optical axis. Therefore, the centers of curvature of the bowed scan lines are also on opposite sides of the optical axis. If all light sources are placed on one side of the optical axis, then all of the scan lines will be imaged on the opposite side of the optical axis. Therefore, the centers of curvature of all of the bowed lines will also lie on same side of the axis. However, each line will be bowed at a different radius of curvature. Thus, this is the source of another type of differential bow.
In single-beam monochrome or single-beam multi-pass color printing systems, a few hundred microns of bow causes no noticeable degradation in the image quality because the bow of the successive scan lines is identical. However, in multi-beam, monochrome, single-station printing systems, or in multi-beam, single-pass color printing systems with single or multiple photoreceptor stations, differential bow causes gross misregistration on the photoreceptor(s) both in the single monochrome image and also among the color layers in the multi-layer color image.
In particular, this misregistration can occur because the magnitude and the earlier described different orientation of the differential bow.
The main shortcoming of the prior art system, as shown in FIG. 5, is its inability to produce scan lines free of differential bow. As indicated above, this poor performance is due to the considerable angular deviation between the chief rays and the system axis between the MCO and the PR image plane.
This angular deviation makes it impossible to establish a reasonable range of workable depth of spot focus that coincides with an acceptable motion compensation range. To state it differently, when a reasonable specified polygon angle tilt (+/-one minute (1") of arc for example), is introduced and the image plane is moved in and out of best focus by reasonable distances (+/-2 mm for example), the variation in spot size along the scan, the amount of differential bow, and the amount of scan line shift due the polygon tilt, each become unacceptable for high quality image generation.